The present invention relates to a method for manufacturing vehicle parts, in particular, parts having high strength of 1500 Mpa or more by using hot stamping.
As the fuel efficiency regulations or safety regulations are strengthened, there is a very high demand for lightweight and high-strength vehicle bodies. As a result, ultra-high strength steel parts with ultimate tensile strength of 1 GPa have been commercialized, and the development of steels having tensile strength of 2 GPa has been recently promoted.
Generally, if a strength of a steel sheet increases, an extension rate is lowered, resulting in a deterioration in processability. One of the technologies proposed for solving this problem is the hot stamping technology. The hot stamping technology was disclosed in British Patent No. 1490535 in the 1970s.
In the hot stamping technology, a steel sheet is heated to a high temperature of, for example, 900° C. or higher and then press-formed and quenched to produce high strength steel part. For the hot stamping, boron steels are used which contain carbon of about 0.2 wt % and use manganese (Mn) and boron (B) as elements for improving heat treatment performance.
Since the hot stamping is performed at high temperature, surface oxidation of steel sheets occurs. In order to solve this problem, aluminum coated steel sheets are proposed. Aluminum coated steel sheets are disclosed in U.S. Pat. No. 6,296,805. A representative example of the aluminum coated steel sheet is Usibor 1500 based on 22MnB5 boron steel.
TABLE 1(Rough composition of 22MnB5, Unit: wt %)CMnSiSCrAlBTi0.21~0.251.10~1.350.15~0.40≤0.0100.10~0.25≤0.0800.0015~0.00400.15~0.045
The hot-stamped parts have problems with trimming. Conventional vehicle parts are cut by using a trimming die, but hot-stamped parts having tensile strength of 1500 Mpa are too strong to cut or pierce by using a trimming die.
In order to cut the hot-stamped parts by using the trimming die, an expensive high-hardness tool steel is required. However, due to frequent damage of the trimming tool, there is a limitation in applying to mass production. Laser is currently used for trimming the hot-stamped parts.
A laser cutter is expensive and productivity of it is relatively low. It takes about 60 seconds to trim a hot-formed body part.
Korean Patent Application Publication No. 2014-0077005 discloses a method for resolving inefficiency of the laser trimming. According to this method, a steel sheet is primarily subjected to shear deformation along a cutting line when formed at 650° C. to 950° C., and then secondarily cut along the cutting line at room temperature.
The method disclosed in Korean Patent Application No. 2014-0077005 is not impossible, but is not suitable for mass production. And further, the configuration of the press forming apparatus becomes complicated, and the edge line by such cutting is not clean. Thus, a post-processing will be required.
Korean Patent Registration No. 1575557 proposes a method for completing trimming while press-forming a steel sheet. A preferable trimming temperature suggested in this patent is 500° C. to 600° C.
The Korean patents are intended to complete trimming partially or completely when the strength of the steel sheet is low before the steel sheet is transformed from austenite to martensite at high temperature. However, according to these patents, the press forming apparatus becomes complicated and cutting quality is not guaranteed.
As of now, the hot-stamped parts having tensile strength of 1500 Mpa are laser-trimmed. The parts that have been formed in the press forming apparatus are loaded in a pallet positioned near the press forming apparatus, cooled to room temperature, transferred together, and then cut in a processing line for laser trimming.